Cobalt-base alloys



United States Patent l 3,333,957 COBALT-BASE ALLOYS Rudolf H. Thielemann, Portland, 0reg., and Harold L. Wheaton, Rolling Meadows, 111., assignors to Martin Marietta Corporation, a corporation of Maryland No Drawing. Filed May 18, 1966, Ser. No. 550,928 7 Claims. (Cl. 75171) This application is a continuatiOn-in-part of our application S.N. 308,349, filed Sept. 12, 1963, and entitled, Cobalt-Base Alloys, now abandoned.

This invention relates to cobalt-base casting alloys having improved strength. More particularly, it relates to cobalt-base alloys which include alloying metal additives capable of imparting to the resultant alloys creep resistance, corrosion resistance and high mechanical work strength at temperatures up to 2100" F., 'i.e., particularly at temperatures of the order of 1800 F. to 2100 F.

In accordance with the present invention, alloys are prepared which comprise a cobalt-base alloy having tungsten, titanium, zirconium, carbon, chromium and tantalun present in proportions and in relationship one to the other, necessary to attain the properties at high temperatures of rupture strength, of creep strength, of resistance to oxidation, of resistance to thermal shock, which properties are of substantially the same magnitude as similar objects of known alloys exhibit at 1600 F. to 1700" F.

Alloys to function properly, for example, as blades, vanes or related parts of jet engines must have high strength, creep resistance, resistance to oxidation, etc. at relatively high temperatures and in oxidizing atmospheres present in the operation ofthe engines. The importance of maintaining strength at elevated temperatures may be illustrated by stating that, if the safe operating temperature of the turbine vanes and blades in aircraft turbojet engines, can be raised by 100 F., i.e., from 1700 F. to 1800 F., the thrust of the engines would be increased appreciably.

One form of prior art cobalt-base alloy is indicated to be a wrought alloy and to have a life of 1000 hours attemperatures of about 1600 F. and at loads of about 10,500 p.s.i. This alloy falls off rapidly in rupture life if either the stress is increased while maintaining a temperature of 1600 F. or if the temperature is increased appreciably above this 1600 F. level. Data presented by the inventor of this alloy shows that one containing 37.9% cobalt, 28.4% nickel, 22.8% chromium, 2.26% tungsten, 2.45% iron, 5.53% titanium and 1.1% carbon, has a rupture life of only 64 hours at 1700 F. and 15,000 p.s.i. stress. Blades for gas turbine engines cast from alloys of this type containing between 3% and 9% of titanium have, up to the present time, not been accepted for use under the high stress, high temperature (about 1800 F.), conditions under which the engines are currently being designed to operate.

Basically, the metal alloy of this invention is comprised by Weight of: from about 15% to 30% of chromium; from about 3% to 15% of tungsten; metal selected from the group consisting of tantalum, columbium or mixtures thereof; the amount of tantalum being from about 1% to 10%; the amount of columbium being from about 0.5% to 5% with the total of tantalum and columbium not exceeding from about 1.55% to 5% of zirconium; from about 0.6% to 3% of titanium; the total amount of zirconium and titanium being in the range between about 3.1% and 6% and the ratio of zirconium to titanium being in the range between 1:1 and 4.5 :1; from about 0.5 to 1.3% of carbon and the remainder being cobalt and incidental impurities; the cobalt content being in the range of 45% to 75%.

Chromium is added to the composition for the purpose of improving strength and oxidation resistance. Chromi- Patented Aug. 1, 1967 um in quantities of about 25% to 30% is optimum for oxidation resistance but above 25 the deleterious embrittling effect of chromium tends to become apparent. Preferably, chromium is used in quantities in the range between about 19% and 24%.

Titanium and zirconium are import-ant elements in determining both the strength and creep resistance of the alloys of this invention at elevated temperatures. Small amounts of titanium have been known to be useful in cobalt-base alloys because of a strengthening effect on the matrix but small amounts of titanium alone, i.e., even up to 1.2% in alloys of the high temperature type herein described improve the rupture strength over that of similar, but zero titanium content alloys, on the order of only 25 and have no apparent effect on creep resistance. Amounts of zirconium alone, even as high as 2%, in the same type of high temperature alloys, while having some enhancing effect on the strength and the creep resistance, does not approach the effect obtained with a combination thereof as taught hereinafter.

In combination, however, in the right amounts and the right proportions, these elements effect an appreciable change in both the rupture strength and the creep resistance. When zirconium and titanium are used in combination, the total amount of titanium and Zirconium should total a minimum of 3.1% and the ratio of zirconium to titanium should be greater than 1:1 in order to show an appreciable increase in rupture strength and creep resistance. When the amount of zirconium and titanium totals less than 3.1%, even though zirconium and titanium are present in ratios of the order required when the total is greater than 3.1% in accordance with this invention, the

alloys when compared to alloys of similar basic components and amounts but having zero zirconium and Zero titanium contents, show if there is any difference, slightly enhanced rupture strengths and creep strengths. For example, when the rupture' life of an alloy of 20.5% chromium, 11% tungsten, 6% tantalum, 0.65% carbon and the balance cobalt is compared to that-of an alloy of 21.2% chromium, 8% tungsten, 4% tantalum, 2.1% zirconium, 0.3% titanium, 1.0% carbon and the balance cobalt, the rupture lives at 2000 F. and 8000 p.s.i. are 46.7 hours and 45.3 hours, respectively, and the creep strengths in inches per inch per hour are 0.001 and 0.0013, respectively.

On the other hand, when the total amount of zirconium and titanium exceeds 3.1% and the ratio of Zirconium to titanium is in the range between about 1:1 and about 4.5 :1, increases in rupture strength and creep resistance can be obtained of the order of 500% when comparing the alloys of this invention with those of similar alloys of 0% zirconium and 0% titanium content. For example, when comparing the alloy referred to above, containing 20.5% chromium, 11% tungsten, 6% tantalum, 0.65% carbon and the balance cobalt with an alloy of the type containing the herein disclosed ratios of zirconium to titanium, for example, the alloy of the hereinafter presented Example VI, which contains 3.2% zirconium, 1.6% titanium, the latter alloy shows a creep resistance in inches per inch per hour of 0.0002 compared to 0.001 for the former and the latter alloy has a rupture life at 2000 F. and 8000 p.s.i. of 213 hours whereas the rupture life of the former is 46.7 hours. Preferably, ratios of zirconium to titanium in the range between about 2:1 and about 4:1, are preferred.

It will be recognized that while operating within the above ratios, the total amount of zirconium plus titanium is subject to variation depending upon such factors as the amount of tungsten and tantalum present. Variations in the amount of tungsten and tantalum within the specified range will necessitate an inverse compensation in the amounts of titanium and zirconium. Below a minimum of zirconium plus titanium of about 3.1%, there is a dropoif in rupture strength. Generally, the total weight of zirconium plus titanium should be in the range between about 3.1% and 6% with total amounts less than 4.75% preferred.

In the alloys of this invention, columbium and tantalum can be used interchangeably, but tantalum alloys are preferred because they tend to exhibit superior oxidation resistance. Columbium and tantalum serve to form stable carbides which impart strength to the alloy. When columbium is substituted for tantalum, the substitution is preferably on an equivalent atomic basis, i.e., 2% by weight of tantalum is replaced by 1% by weight of columbium.

Carbon is required in this alloy in quantities to produce stable carbide particle hardening. The strength of the alloy is highest when the carbon content is above about .8% and in proper proportions for combination with the carbide forming elements.

It has been found that if the boron content of the alloy exceeds 0.2%, then the high temperature properties are deleteriously affected.

Molybdenum is an optional element in this alloy and may only be present without deleteriously affecting the properties of the alloy if the amount thereof does not exceed 3 by weight of the alloy.

While silicon, manganese, and iron are not essential ingredients of the alloys of the present invention, it has been found that the addition of small percentages of any one or more of these ingredients, that is, up to about 1% of silicon, up to about 2% of manganese, and up to about 5% of iron, may in certain instances somewhat improve certain properties of the alloy. However, when seeking the highest rupture strength, these elements should be kept as low as possible.

Tungsten is most effective in this alloy when present in amounts between about 6% and about and the ratio of tungsten to tantalum generally within the broad range of 1:1 and 3.5:1 and preferably 1.5:1 and 3:1.

A preferred range of proportions of constituents of the alloy of this invention in percentages by weight is as follows:

From about 19% to 24% of chromium; from about 6% to 10% of tungsten; from about 3% to 8% of tantalum or from 1.5% to 4% of columbium or mixtures of tantalum and columbium which do not exceed about 8% by weight of the alloys; from about 0.75% to 2.0% of titanium; from about 2.0 %to 4.0% of zirconium; the total amount of zirconium and titanium being in excess of 3.1%; from about .8% to 1.1% of carbon and from about 55% to 65% of cobalt. This preferred composition may contain about 0.1% maximum boron, about 1% maximum iron, about 0.2% maximum of silicon and about 0.5% maximum of molybdenum and about 2.0% maximum of nickel. The minor alloying constituents should not be present in a total amount exceeding about 2.0%.

The following are examples of the preparation and test results of various formulations of the cobalt-base alloy of this invention, in the approximate weight percentages indicated.

Example I A pound alloy melt of a cobalt-base metal alloy composition consisting essentially of 21% of chromium, 10% of tungsten, 5% of tantalum, 1% of iron, 1.5% of titanium, 3% of zirconium, 1.0% of carbon and the balance essentially cobalt all by weight, was prepared by melting a mix in a magnesia crucible under high vacuum conditions.

A cluster of 12 test bars was formed from the 15 pound melted alloy by the usual investment casting technique under high vacuum conditions. These bars were each 3" long and A" in diameter.

At room temperature, the test bars had at rupture an elongation of about 6 percent and a tensile strength of 115,000 p.s.i.

The test bars had an elongation of 19 percent with a rupture life of approximately 35.7 hours under a load of 17,500 p.s.i. at a temperature of 1800 F. in air and an elongation of 11 percent with a rupture life of 213.3 hours under a load of 8,000 p.s.i. at a temperature of 2000 F. in air.

Example [I A 15 pound alloy melt and test bars of the same dimensions as set forth in Example I of a cobalt-base metal alloy composition containing 22% of chromium, 4.7% of tungsten, 2.3% of tantalum, 1.5% of titanium, 1% of iron, 4.4% of zirconium, 1% of carbon and the balance essentially cobalt, all by weight were prepared in the same manner as set forth in Example I.

The test bar-s had an elongation of 30 percent with a rupture life of approximately 29.8 hours under a load of 17,500 p.s.i. at a temperature of 1800 F. in air and an elongation of 17 percent with a rupture life of 293.6 hours under a load of 8,000 p.s.i. at a temperature of 2000 F. in air.

Example. III

A 15.pound alloy melt and test bars of the same dimensions as set forth in Example I of a cobalt-base metal alloy composition containing 21% of chromium, 8% of tungsten, 4% of tantalum, 1.20% of titanium, 2.2% of zirconium, 0.8% of carbon and the balance essentially cobalt, all by weight were prepared in the same manner as set forth in Example I.

At room temperature the test bars had at rupture an elongation of 3.5 percent and a tensile strength of 98,000 p.s.i.

The test bars had an elongation of 18 percent with a rupture life of approximately 60.6 hours under a load of 17,500 p.s.i. at a temperature of 1800 F. in air and an elongation of 8 percent with a rupture life of 190.4 hours under a load of 8,000 p.s.i. at a temperature of 2000 F. in air.

Example IV A 15 pound alloy melt and test bars of the same dimensions as set forth in Example I of a cobalt-base metal alloy composition containing 22.6% of chromium, 6.5% of tungsten, 3.2% of tantalum, 1.5% of titanium, 3.5% of zirconium, 0.45% of iron, 0.92% of carbon and the balance essentially cobalt, all by weight were prepared in the same manner as set forth in Example I.

At room temperature the test bars had at rupture an elongation of 4.5 percent and a tensile strength of 106,000 p.s.i.

The test bars had an elongation of 11 percent with a rupture life of 205.6 hours under a load of 8,000 p.s.i. at a temperature of 2000 F. in air.

Example V A 15 pound alloy melt and test bars of the same dimensions as set forth in Example I of a cobalt-base metal alloy composition containing 21.5% of chromium, 9.0% of tungsten, 2.25% of columbium, 0.8% of titanium, 2.5% of zirconium, 1.0% of carbon and the balance essentially cobalt all by weight were prepared in the same maner as set forth in Example I.

The test bars had an elongation of 17 percent with a rupture life of 305 hours under a load of 17,500 p.s.i. at a temperature of 1800 F. in air and an elongation of 4 percent with a rupture life of 163 hours under a load of 8,000 p.s.i. at a temperature of 2000 F. in air.

Example VI A 15 pound alloy melt and test bars of the same dimensions as set forth in Example I of a cobalt-base metal alloy composition containing 20.3% of chromium, 9.9% of tungsten, 5.2% of tantalum, 3.2% of zirconium, 1.6% of titanium, 1.0% of carbon, and the balance essentially cobalt, all by weight were prepared in the same manner as set forth in Example I.

The test bars had a creep strength in inches per inch per hour of 0.0002 and a rupture life of 213 hours under a load of 8,000 p.s.i. at a temperature of 2000 F. in air.

Example VII A 5 pound alloy melt and test bars of the same dimensions as set forth in Example I of a cobalt-base metal alloy composition containing about 20.8% of chromium, about 11.8% of tungsten, about 6.1% of tantalum, about 1.2% of titanium, about 0.80% of carbon, and the balance essentially cobalt, all percentages by weight were prepared in the same manner as set forth in Example I.

The test bars of this example had a creep strength in inches per inch per hour of 0.001 and a rupture life of 62 hours under a load of 8,000 p.s.i. at a temperature of about 2000 F. in air.

Comparison of the alloys of Examples I to VI with the alloy of Example VII, shows that alloys containing zirconium and titanium, in total amounts and in proportions outside of the claimed ranges are inferior in rupture life and creep strength.

The above detailed description of this invention is intended to be merely illustrative. No unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

We claim:

1. A metal alloy consisting essentially of by weight, from about 15% to 30% of chromium, from about 3% to 15 of tungsten, metal selected from the group consisting of tantalum, columbium and mixtures thereof, said tantalum being present in amounts from about 1% to said columbium being present in amounts from about 0.5% to 5% with the maximum amount of tantalum plus columbium totaling about 10%, from about 1.55% to 5% of zirconium, from about 0. 6% to 3% of titanium, the total amount of zirconium and titanium being in the range between 3.1% and 6% and the ratio of zirconium to titanium being in the range between 1:1 and 45:1, from about 0.5% to 1.3% of carbon, and the remainder being cobalt and incidental impurities, the cobalt content being in the range between 45% and 75%.

2. A metal alloy consisting essentially of by weight, from about to 30% of chromium, from about 3% to 15% of tungsten, a metal selected from the group consisting of tantalum, columbium and mixtures thereof, said tantalum being present in amounts from about 1% to 10%, said columbium being present in amounts from about 0.5% to 5% with the maximum amount of tantalum plus columbium totaling about 10%, from about 1.55% to 5% of zirconium, from about 0.6% to 3% of titanium, the total amount of zirconium and titanium being in the range between 3.1% and 6% and the ratio of zirconium to titanium being in the range between 1:1 and 4.5 :1, from about 0.5% to 1.3% of carbon, up to 0.2% boron, up to 5% iron, up to 3% molybdenum, up to 10% nickel, and the remainder being cobalt and incidental impurities, the cobalt content being in the range between 45% and 75%.

3. A metal alloy consisting essentially of by weight from about 19% to 24% of chromium; from about 6% to 10% of tungsten, a metal selected from the group consisting of tantalum, columbium and mixtures thereof, said tantalum being present in amounts from about 3% to 8%, said columbium being present in amounts from about 1.5% to 4% with the amount of tantalum and columbium being less than a total of about 8%, from about 2.0% to 4.0% of zirconium; from about 0.75% to 2.0% of titanium; the total amount of zirconium and titanium being in excess of 3.1% and the ratio of zirconium to titanium being in the range between 2:1 and 4: 1, from about .8% to 1.1% of carbon and the remainder being cobalt and incidental impurities, the cobalt content being in the range between 55% and 4. A metal alloy consisting essentially of by weight from about 19% to 24% of chromium; from about 6% to 10% of tungsten, a metal selected from the group consisting of tantalum, columbium and mixtures thereof, said tantalum being present in amounts from about 3% to 8%, said columbium being present in amounts from about 1.5 to 4% with the amount of tantalum and columbium being less than a total of about 8%, from about 2.0% to 4.0% of zirconium; from about 0.75 to 2.0% of titanium; the total amount of zirconium and titanium being in excess of 3.1% and the ratio of zirconium to titanium being in the range between 2:1 and 4: 1, up to 0.1% of boron, up to 1% of iron, up to 0.2% of silicon, up to 0.5% of molybdenum and up to 2.0% of nickel; from about .8% to 1.1% of carbon and the remainder being cobalt and incidental impurities, the cobalt content being in the range between 55% and 65%.

5. A metal alloy consisting essentially of by weight, about 21% of chromium, about 10% of tungsten, about 5% of tantalum, about 3% of zirconium, about 1.5% of titanium, about 1% of carbon, the remainder being cobalt and incidental impurities.

6. A metal alloy consisting essentially of by weight, about 22.6% of chromium, about 6.5% of tungsten, about 3.2% of tantalum, about 3.5% of zirconium, about 1.5% of titanium, about 0.92% of carbon, about 0.45% of iron, the remainder being cobalt and incidental impurities.

7. A metal alloy consisting essentially of by weight, about 21.5% of chromium, about 9.0% of tungsten, about 2.25% of columbium, about 2.5% of zirconium, about 0.8% of titanium, about 1.0% of carbon, and the balance essentially cobalt.

References Cited UNITED STATES PATENTS 6/1941 Rohn et a1. 171 3/1961 Thielemann 75-171 

1. A METAL ALLOY CONSISTING ESSENTIALLY OF BY WEIGHT, FROM ABOUT 15% TO 30% OF CHROMIUM, FROM ABOUT 3% TO 15% OF TUNGSTEN, METAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM, COLUMBIUM AND MIXTURES THEREOF, SAID TANTALUM BEING PRESENT IN AMOUNTS FROM ABOUT 1% TO 10%, SAID COLUMBIUM BEING PRESENT IN AMOUNTS FROM ABOUT 0.5% TO 5% WITH THE MAXIMUM AMOUNT OF TANTALUM PLUS COLUMBIUM TOTALING ABOUT 10%, FROM ABOUT 1.55% TO 5% OF ZIRCONIUM, FROM ABOUT 0.6% TO 0% OF TITANIUM, THE TOTAL AMOUNT OF ZIRCONIUM AND TITANIUM BEING IN THE RANGE BETWEEN 3.1% AND 6% AND THE RATIO OF ZIRCONIUM TO TITANIUM BEING IN THE RANGE BETWEEN 1:1 AND 4.5:1, FROM ABOUT 0.5% TO 1.3% OF CARBON, AND THE REMAINDER, BEING COBALT AND INCIDENTAL IMPURITIES, THE COBALT CONTENT BEING IN THE RANGE BETWEEN 45% AND 75%. 